Restriction of dietary calories reduces cancer formation in experimental animals and probably also in humans. This effect is generally attributed to the inhibitory effect of fasting on cell proliferation. Here we studied the effect of fasting on physiological cell death through apoptosis by using rat liver as a model. (i) In normal liver, involution of hyperplasia by apoptosis was reinforced by food withdrawal and suppressed by feeding. Complete food withdrawal for 8 days or food reduction by 40% for 3 months eliminated 20-30% of normal liver cells through apoptosis. (ii) Putative preneoplastic liver foci exhibited severalfold higher rates of DNA replication and apoptosis than unaltered liver. Food restriction lowered DNA replication but increased apoptosis, which reduced the number and volume of putative preneoplastic liver foci by 85% within 3 months. Subsequent return to ad libitum feeding normalized cell replication and apoptosis but clear differences in the volume and number of putative preneoplastic liver foci persisted throughout the following 17 months. Treatment of animals after food restriction with nafenopin, a peroxisome proliferator and potent tumor promoter, produced only half as many hepatocellular adenomas and carcinomas as in animals fed unrestrictedly throughout their lifetime. This indicates that food restriction had actually eliminated a part of the initiated cells. This study demonstrates that food restriction preferentially enhances apoptosis of preneoplastic cells. This effect in combination with lowered cell replication provides protection from carcinogenesis.